1. Field of the Invention
The present invention relates to electric power distribution systems, and more particularly to a method and apparatus for detecting series and/or parallel arc faults for electric power systems.
2. Description of the Related Art
Electrical systems used in complex environments such as aerospace systems, industrial environments, vehicles, and residential environments include a large number of electrical circuits, devices, and wires. Arc faults may occur in any of the electrical circuits, or along the wires. If not detected promptly, arc faults may cause short circuits, malfunctions, and fires in the equipment serviced by the electrical circuits or wires exhibiting arc faults.
Arc fault detection and protection pose a significant challenge in such complex environments. Correct and prompt arc fault detection and protection are critical in aircraft environments. Airlines, aircraft manufacturers, the military, and regulatory agencies such as the FAA have expressed the need for accurate and fast arc fault detection and protection systems. A generalized method and system that can reliably detect and prevent series and/or parallel arcs in both AC and DC electric power systems is needed.
Prior art methods have focused research and technology development efforts mostly on the detection of parallel arc faults. Parallel arc faults exhibit high energy levels and are easy to differentiate from other operating conditions. In contrast, series arc fault currents have low energy levels, being limited in magnitude by the electrical load along which they occur. Hence, series arc faults are more difficult to detect. Conventional circuit breakers, currently in widespread use in the aerospace and general residential and industrial environments, are designed to detect only over-current and overload conditions. Numerous serious or fatal electrical incidents are caused by low energy level arc fault conditions resulting from damaged or aging wire. Such low energy level arc faults are not typically detected by conventional circuit breakers.
The aging of wire systems is recognized as a significant safety concern in commercial aviation. Aging wires are a source of arc faults. The problem of aging wire systems extends to virtually all electrical systems in homes and other buildings, consumer products, nuclear power plants, public transit systems, and railroad systems.
Currently there are no systematic means to distinguish a series arc signal from normal and abnormal transient signals, or from various load profiles that occur in a typical AC or DC power distribution system. The existing arc fault detection approaches use a combination of filtering and threshold detection to distinguish such arcs from normal signal occurrences. Such methods are not universal, and are susceptible to frequent nuisance trips. The choice of filtering strategy depends on the nature of the distribution network environment and on the characteristics of the load. However, the nature of the distribution network environment and the characteristics of the loads in an electrical environment may not necessarily be known at the time when generalized libraries are created, such libraries being used to differentiate between arc faults and normal/abnormal transient characteristics of the loads.
A few publications have studied arc detection techniques for electrical power systems. One such technique is described in U.S. Pat. No. 5,561,605, entitled “Arc Detection Using Current Variation”. In the technique described in this work, notch filters are used to remove the fundamental and harmonic components from an AC line current and voltage. The resultant signal (current or voltage) is subtracted from the value obtained from a previous period. Normally this difference signal is zero, however, under certain arc fault conditions, the difference can be non-zero. The absolute value of the difference signal is used to create a pulse train whose frequency depends on the magnitude of the error signal. A pulse count is accumulated in a synchronous summer over a predefined time. A trip signal is generated if the accumulated pulse count exceeds a specific value over the predefined time. This technique, however, does not detect arc faults with small absolute signatures.
A disclosed embodiment of the application addresses these and other issues by utilizing a generalized arc fault detection system that detects series and parallel arcs in real time in both AC and DC electrical systems; differentiates arc faults from normal loads both in steady state and during normal and abnormal electric power system transients; allows detection of small arc currents in the presence of large load currents; and provides excellent trip-free operation and noise immunity against switching devices and potential cross-talk among adjacent power lines.